Fused silica is highly relevant to such applications as windows and mirrors used in outer space, and increasingly, it is becoming relevant to optical elements for deep ultraviolet photolithography. However, it is generally known that prolonged exposure of fused silica to intense deep ultraviolet radiation of the type utilized in photolithography leads to optical damage which is generally manifested in the form of changes in the optical and physical properties of the glass.
One form of optical damage observed in fused silica glass is a physical densification or compaction of the exposed regions of the glass. Compaction is generally observed by interferometry where the alteration of the optical phase front is measured through the damaged regions, and is reported as optical path difference, the product of refractive index and path length, in ppm or waves of 633 nm light. Thus, the optical phase front of stepper lens elements for photolithographic applications which utilize deep ultraviolet wavelengths at 193 and 248 nm for high resolution microcircuit fabrication may become altered due to optical modification as a consequence of prolonged exposure. Even though small changes in the optical phase front produced by the effect of exposure over the life of the lens barrel are expected, at present the maximum acceptable change is not known. What is known however, is that there is a relationship between alterations in fused silica and the ultimate effect of such changes on the wavefront.
The question of what factors contribute to the propensity of various silica materials to optical damage when exposed to high energy laser irradiation is not settled and several possible answers have been advanced in the literature.
Recently, in co-assigned, co-pending PCT patent application Ser. No. PCT/US97/11697, deposited Jul. 1, 1997, titled "Fused Silica Having High Resistance to Optical Damage," it was suggested that radiation-caused optical damage can be minimized or eliminated by precompacting fused silica by such processes as hot isostatic pressing and by high energy pre-exposure in order to thereby desensitize the glass to subsequent high energy irradiation during actual use.
There continues to be a need for laser damage resistant fused silica. Accordingly, it is the object of the present invention to provide a fused silica stepper lens which is resistant to laser-induced damage, specifically defined as densification.